Steam co-injection for the reduction of heat exchange and furnace fouling

ABSTRACT

A process for removing or reducing the accumulation of foulant within furnaces and heat exchangers in industrial systems such as an oil refinery by introducing a periodic steam blast. The steam blast is directed into the fluid stream from which the foulants form on to the heat exchanger surfaces. The steam blast increases the flow rates, creates turbulence and increases the temperature within the heat exchanger to dislodge foulant in both a soft and hardened states from internal surfaces upon which foulants have adhered and accumulated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims thebenefit of and priority to U.S. Provisional Application Ser. No.63/124,336 and U.S. Provisional Application Ser. No. 63/124,364, both ofwhich were filed on Dec. 11, 2020, and entitled “Steam Co-Injection forthe Reduction of Heat Exchange and Furnace Fouling” and are herebyincorporated by reference their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

This invention relates to removing and preventing the buildup of solidsthat form inside heat exchanger pipes and other conduits in a refinery.

BACKGROUND OF THE INVENTION

Fouling is a broad term that describes the accumulation of unwantedmaterial on solid surfaces that impairs, impedes or interferes with thefunction of systems or equipment. In refineries, much effort is expendedto control fouling.

Fouling in heat exchangers and furnaces is well known in oil refiningand much effort has gone toward understanding it and trying to controlor abate it. Crude oil includes an array of components that vary fromone crude source to another and some of those components are vulnerableto the inducing of fouling. Fouling precursors include viscous polymericmaterial and mesophase along with other crude oil components. Theseprecursors create dense and a well-adhered foulant layer on the surfacesof pipes and conduits which can also trap solid particles such as saltcrystals or corrosion products. Heating causes various foulingprecursors to form or precipitate from crude oil within the heatingsystems and, in order to refine crude oil, the crude oil must be heated.Typically, crude oil is heated through a succession of heat exchangersin preparation for distillation and other refining processes. In time,the congealed and solidified foulants harden and become strongly adheredto heat exchange surfaces in baked-on-like carbon deposits that are verydifficult to remove by solvents, scrubbing and scraping.

Efforts to control fouling include crude oil blending to keep highfouling crudes from being refined without dilution with lower foulingcontent crudes and adding fouling retardants. Ultimately, foulingoccurs, and the next step is shutting down equipment to remove thefoulant or replacing parts within the process equipment to renew theperformance of the furnace or heat exchanger. Shutting down equipmentimpairs financial performance of a refinery as does operating withimpaired equipment and eventually forcing a decision to shut down mustbe made.

It has been reported that fouling may (but not always) be minimized bymaintaining a relatively high (for example, 2 m/s) and uniform fluidvelocity throughout the fouling prone components, where stagnant regionsneed to be eliminated. Components are normally overdesigned toaccommodate the fouling anticipated between cleanings. However, asignificant overdesign can be a design error because it may lead toincreased fouling due to reduced velocities. Periodic on-line pressurepulses and/or backflow have been suggested if the capability iscarefully incorporated at the design time. Low-fouling surfaces (forexample, providing a very smooth surface perhaps implanted with ions, orof low surface energy material like Teflon®) may be an option for someapplications. Modern components are typically required to be designedfor ease of inspection of internals and periodic cleaning. On-linefouling monitoring systems are designed for some application so thatoffline blowing or cleaning can be applied before an unexpected shutdownis necessary or the system is damaged or compromised.

The industry has long needed an effective foulant control scheme thatallows continued refinery operation but reduces or better yet, removesaccumulated fouling deposits from within areas prone for such problems.

BRIEF SUMMARY OF THE DISCLOSURE

The present embodiment relates to a system for heating a process fluidstream where the process fluid includes components that are prone tocausing fouling or forming deposits on one or more surfaces within theindustrial device. The system includes a housing for circulating heatingfluid and a number of tubes arranged to pass through the housing. Thetubes have a peripheral wall with an inside and an outside where theinside of all of the tubes is characterized as a tube-side chamber. Thetubes are arranged for carrying process fluid as it is heated indirectlythrough conduction through the peripheral wall and the housing and tubeside chamber are sealed from one another. The system includes both aheating fluid source and a steam source although one source could supplyboth the heating fluid and steam. A conduit is arranged to supply thesteam from the steam source to the inside of the tubes having a valvearranged in the conduit to permit steam from the steam source to passthrough the tube side chamber and also arranged to close off steam frompassing through the tube side chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefitsthereof may be acquired by referring to the following descriptions takenin conjunction with the accompanying drawings in which:

FIG. 1 is a schematic elevation view of a conventional shell and tubeheat exchanger modified to perform the process of the present invention;

FIG. 2 is chart showing thermal resistance caused by fouling over time;

FIG. 3 depicts a diagram of the test rod used in experimentation;

FIG. 3 depicts an enlarged photo (1) of the inside of the test rod atposition 1 showing foulant and bare metal;

FIG. 3 depicts an enlarged photo (2) of the inside of the test rod atposition 2 showing additional foulant;

FIG. 3 depicts an enlarged photo (3) of the inside of the test rod atposition 3 showing further foulant on the inside of the test rod;

FIG. 3 depicts an enlarged photo (4) of the inside of the test rod atposition 4 showing the scoured inside of the test rod;

FIG. 4 is a chart showing furnace inlet temperature of crude oil in arefinery after being subjected to twelve hours of water carryover thatformed a steam treatment within the heat exchanger tubes and showedimproved heat transfer once the water carryover was suspended; and

FIG. 5 shows an alternative heat exchanger embodiment where a furnace isheating the process fluid or, more particularly, heating the crude oil.

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement orarrangements of the present invention, it should be understood that theinventive features and concepts may be manifested in other arrangementsand that the scope of the invention is not limited to the embodimentsdescribed or illustrated. The scope of the invention is intended only tobe limited by the scope of the claims that follow.

As shown in FIG. 1, a shell and tube heat exchanger 10 which includes aheader 12 with a tube sheet 20. The header 12 is divided into twoplenums by a divider 13 where an inlet plenum 14 is arranged in theupper portion of the header 12 and an outlet plenum 16 is in the lowerportion of the header 12. A tube bundle 22 is shown where each of theplurality of tubes 23 are arranged to have two long straight generallyhorizontal runs with an upper run extending away from the inlet plenum14 at the tube sheet 20 turning a tight, but rounded or arcuate 180degree turn carrying back through a lower run to the lower outlet plenum16. The tube sheet 20 includes a number of individual openings thereinwhere each of the tubes 23 are affixed, typically by welding, such thatfluid is carried inside the tubes from the inlet plenum 14 along thegenerally horizontal runs back to the outlet plenum 16. As will beexplained below, the fluid inside the tubes 23 will be heated. Forclarity, the tubes may be described as being in fluid communication withthe inlet and outlet plenums 14 and 16 and the internal volume insidethe tubes 23 may be described as the tube-side chamber of the heatexchanger 10. In addition, the inlet and outlet plenums 14 and 16 arealso sealed from one another inside the header 12 so any fluid enteringthe outlet plenum 16 is arranged, in operation, to come through one ofthe tubes 23 in the tube bundle 22 and thereby not directly from theinlet chamber 14 such that fluid would bypass the tube bundle 22.

The heat exchanger 10 further includes a shell 25 that attaches to theheader 12 near the tube sheet 20, typically by bolting, to close andcover the outside of the tube bundle 22 to define a shell-side chamber26 under the shell 25 and around the outside of the tubes 23. The shell25 is shown with a generally cylindrical shape with a bulbous, generallyhemispherical end. One advantage of this generally cantileveredarrangement where everything is basically fixed at the left end (as seenin the drawing figure) and unrestricted at the other end is that thetubes 23 and shell 25 are generally free to expand and contract as thetemperature of the heat exchanger 10 increases and decreases intemperature.

The tubes 23 may include supporting brackets to keep them spaced fromone another and allow heat exchanging fluid to flow between all of thetubes and thereby surround the outside surface of all of the tubes 22rather than permit any tubes to sag together and reduce the contact areaon the outside of the tubes 23 for heat to cross from fluid in theshell-side chamber to the fluid within the tubes 23.

A process fluid inlet 15 is attached to the header 12 to deliver aprocess fluid into the inlet plenum 14 from a pipe or conduit (notshown). A process fluid outlet 17 is similarly attached to the header 12and arranged to receive the process fluid from the outlet plenum 16 anddeliver it on to another pipe, conduit or system. The supply of theprocess fluid is well known and plumbing in the fluid supply and takeaway is not part of the invention. However, typically the heat exchanger12 is operated to keep the tube-side chamber in a fluid full arrangementwithout air or gas pockets. In the case of a refinery, the process fluidis typically a crude oil or a petroleum intermediate product that is tobe heated for some refining process.

The shell 25 includes a shell-side inlet 27 and a shell-side outlet 28to circulate heat exchange fluid in and out of the shell-side chamber26. To heat the process fluid, a second hotter fluid is brought intocontact with the outside of the tubes 23 by delivering the hot, secondfluid to the shell-side inlet 27 where the hot, second fluid fills theshell-side chamber 26 under the shell 25 providing its heat to theprocess fluid by contact with the exterior surfaces of the tubes 23. Thesecond fluid may be steam or another fluid including hot liquids such asa petroleum stream that has been previously heated in a prior processsystem. The second fluid flows through the shell-side chamber 26 to asecond fluid outlet 28 attached to the shell 25 where it is carried awayin a conduit. Like the tube-side chamber, the shell-side chamber 26 istypically operated to be fluid full.

Turning now to the modifications to the shell and tube heat exchanger10, for the purposes of the present invention, the inlet plenum 14additionally includes a steam inlet 30 to receive steam into the inletplenum 14. Steam is supplied by a steam source 31 and the delivery iscontrolled by steam valve 32 in the steam line from source 31 to steaminlet 30. The steam valve 32 may be opened and closed by an operator ormay be operated by a control device 33 that may include a timer forperiodically opening the steam valve 32 at a set time and for apredetermined time. The control device may also include logic to respondto temperature as measured in the inlet and outlet plenums to use thetemperature difference to lead to a steam blast treatment when athreshold temperature difference is detected. In the present invention,steam is periodically injected or blasted into the inlet plenum withforce and in a manner and volume to dislodge foulants in the heatexchanger 10 and most particularly adhering in the tube-side chamber.The steam enters with force and in such a volume as to carry through thetubes 23 all the way to or close to the outlet plenum 16 and possiblythrough to the tub-side outlet 17. Prior to the steam being injected,the fluid flow through the tube-side volume is stable although eddiesand other non-linear flow characteristics are likely present. But uponthe commencement of steam injection, much turbulence is introduced.Moreover, the temperature of the tube-side fluid increases which mayslightly alter the dimension of the tubes by thermal expansion. Allthree of these phenomena may be at play removing foulant and foulantprecursors.

Fouling on heat exchanger surfaces reduces heat transfer efficiency,decreases heat flux, induces corrosion under deposits, increases the useof cooling water and increases metal skin tube temperature whichpromotes degradation of the metallurgy, or fixes an upper temperaturelimit to the process, beyond which a shutdown of the equipment isrequired for cleaning. As fouling deposits accumulate, piping or flowchannels form which reduces flow, increases pressure drop, increasesupstream pressure, increases energy expenditure, may cause flowoscillations, slugging in two-phase flow, cavitation, may increase flowvelocity elsewhere, may induce vibrations, may cause flow blockage, andsets an upper hydraulic limit on the process, beyond which a shutdown ofthe equipment is required for cleaning.

For the present invention, timing and duration of the periodic steaminjection may include several considerations including the transitoryreduction of throughput of the process fluid. The steam may beco-injected with the process fluid however the flowrate of the fluid mayhave to be reduced to accommodate the steam volume. There will be anaccompanying economic impact to the transitory reduction of throughput.An instantaneous increase in pressure will also accompany the steampulse, and thus the hydraulic limit of the system will need to beconsidered. Since the pulse will be injected as steam, there will not bean instantaneous volume expansion that would accompany a water pulsesubject to rapid evaporation, however the steam will need to be injectedabove the system pressure to ensure it is directed through theappropriate flow path.

The steam pulsing program would ideally be conducted with daily orweekly frequency depending on the severity of the fouling. Starting witha freshly cleaned heat exchanger, the steam pulsing is expected to havegreater utility against soft deposits, and thus the pulses should beused as frequently and as early in the run as is practical. Thermalaging of foulant produces a harder deposit that can have a harder bondto the underlying metal substrate making overall fouling management moreeffective when deposits are removed before they are allowed tosubstantially age. An effective steam pulse would displace up to 2volumes of process liquid from the heat exchanger and last forapproximately 1 minute. This short, sharp pulse of steam would minimizethe amount of water that would dissolve into the process fluid, therebymaximizing its effectiveness at removing foulant.

In general, the targeted conditions for implementing the invention is touse superheated steam at about 500° F. and at about 100 psig which wouldhave a specific volume of about 4.85 ft³/lb. In a heat exchanger having500 one-inch diameter tubes where each tube has an effective length ofabout 50 feet, the internal volume of the tube bundle would be 136.3ft³. In the steam blast condition, two volumes of the heat exchangertube bundle would be displaced in one minute at a mass flow rate ofapproximately 3,400 lbs/hr of superheated steam. In normal operatingconditions, about 450-1,250 lbs/hr per tube bundle pass are typicalflowrates of velocity steam in a coker furnace.

EXAMPLES

Fouling tests were done with crude oil both with and without 10 vol %water being added. The fouling tests were completed in a thermal foulingtester, which consists of a flow loop where oil is directed over a hotmetal surface that is held at a constant elevated temperature (450° C.)above the liquid temperature (50° C.). As foulant formed on the surface,heat transfer was impeded, and the outlet temperature of the oildecreased. The fouling thermal resistance was evaluated during theexperiment as the inverse of the overall heat transfer coefficient whichcan be calculated from experimental parameters.

In one arrangement, a volume of crude oil was delivered into a heatexchanger with 10 percent by weight water added. The water was rapidlyvaporized into steam within the heat exchanger. After the test volume ofcrude oil with water had fully passed through the heat exchange, theheat duty of the hot preheat train increased suggesting that the steamin the system had removed some foulant or prevented its deposition andimproved heat transfer.

FIG. 2 shows the results of the laboratory fouling tests with steam. Inthe graph thermal resistance caused by the foulant is plotted againsttest time. The initial test with neat crude had a rapid fouling ratewhich increased the thermal resistance over time as indicated by thesteep slope of the first portion of the curve. Once the neat crude testended, the system was recharged with the crude oil that had beenemulsified with 10 vol % water. In an effort to see if foulant could beremoved by the vaporization of water during the test, the same test rodwas used for all tests, and was therefore already fouled at the onset ofthe second test, which contained the water phase. A natural sluggingeffect occurred in the flow system, which meant water was introducedintermittently and generated steam pulses that lasted approximately 3-4minutes each and occurred approximately every 15 minutes during the2-hour test. The fouling curve for the second test in FIG. 2 shows thatthe fouling rate was reduced by a factor of 5 when steam was in thesystem. Vaporization of the water was confirmed visually through thesite glass in the flow system.

FIG. 3 shows optical microscopy images of the fouled rod after the 2fouling tests. Evidence of foulant removal along the length of the testrod is evident. The arrows in FIG. 3 highlight the foulant and thestriations shown in FIG. 3 (Photo 4) were likely caused in the foulantby the turbulent two-phase flow of the crude oil and steam. Thelocations of each of the images along the length of the 3-inch testsection of the rod are shown in FIG. 3. The flow striations in FIG. 3(Photo 4) are especially indicative of the power of high velocity steampulses within a tube to remove foulant from a fouled surface simplythrough the shear force of turbulence. The turbulence of steamintroduced into the tubes 23 in the gas phase are powerful for removingfoulant and fouling precursors.

FIG. 4 shows results from a water carryover event in a refinery wherewater was carried into a heat exchanger in an emulsion state with crudeoil. The graph in FIG. 4 shows the crude furnace inlet temperatureplotted against time. Prior to the water carryover, the furnace inlettemperature was approximately 595° F. Water carryover occurred forapproximately 12 hours, and after this, the furnace inlet temperatureincreased by 14. This increase indicates a recovery in heat duty of 10MM BTU/hr for the hot preheat train.

Turning to FIG. 5, another type of heat exchanger used to heat crude oilis a furnace 110 such as schematically shown. The Figure includes ahousing 128 with a flame or heat source 127 heating up the inside 126 ofthe furnace 110. Passing through the inside 126 is a tube bundle 122comprising a number of tubes 123. Crude oil is directed through the tubebundle from inlet plenum 114 as the crude is delivered from conduit 115and taken out at plenum 116 and delivered away through conduit 117.Periodically, a steam blast is delivered into the inlet plenum andcarried through the tube bundle 122 via steam inlet 130 from steamsource 131. Steam is typically a standard utility in a refinery as it isused and created in multiple systems. The delivery of the steam iscontrolled by valve 132 that is under the management of the controlsystem 133. This is basically a similar system to that shown in FIG. 1with similar elements provided with similar numbering, but with “100”added to the numbers. And the inside 126 of the furnace 110 is analogousto the shell side chamber 26 of the heat exchanger in FIG. 1.

Ultimately, the steam pulses are preferred to be short and sharp andaround 100 to 500 psig where the upper number is considered relative tothe robustness of the tubes and heat exchange structure. The steamshould be at least 50 psi above the pressure of the process fluid andlast from about 1 to 30 seconds although a broader blast may be imposed.In crude oil, adding water, especially water that may form an emulsionis strongly un-preferred as water and steam tend to cause problemselsewhere downstream of the heat exchangers and furnaces. So, theaddition of steam is done as needed to control fouling, but at leastsomewhat sparingly as the steam or water is preferably knocked outdownstream, but the volume of water knockout systems is typically notgreat. It is contemplated that steam may be used as both the heatingfluid and for foulant removing.

In closing, it should be noted that the discussion of any reference isnot an admission that it is prior art to the present invention,especially any reference that may have a publication date after thepriority date of this application. At the same time, each and everyclaim below is hereby incorporated into this detailed description orspecification as an additional embodiment of the present invention.

Although the systems and processes described herein have been describedin detail, it should be understood that various changes, substitutions,and alterations can be made without departing from the spirit and scopeof the invention as defined by the following claims. Those skilled inthe art may be able to study the preferred embodiments and identifyother ways to practice the invention that are not exactly as describedherein. It is the intent of the inventors that variations andequivalents of the invention are within the scope of the claims whilethe description, abstract and drawings are not to be used to limit thescope of the invention. The invention is specifically intended to be asbroad as the claims below and their equivalents.

1. A system for heating a process fluid stream where the process fluidincludes components that are prone to causing fouling or formingdeposits on one or more surfaces within the industrial device, where thesystem comprises: a) housing for circulating heating fluid; b) aplurality of tubes arranged to pass through the housing where the tubeshave a peripheral wall with an inside and an outside where the inside ischaracterized as a tube-side chamber and the tubes are for carryingprocess fluid as it is heating indirectly through conduction through theperipheral wall and the housing and tube side chamber are sealed fromone another; c) a source of heating fluid; d) a steam source; e) aconduit from the steam source to the inside of the tubes having a valvearranged to permit steam from the steam source to pass through thetube-side chamber and also arranged to close off steam from passingthrough the tube-side chamber.
 2. The system according to claim 1wherein the steam valve is connected to a control system that isarranged to periodically open and close the steam valve.
 3. The systemaccording to claim 2 wherein the control system includes a timer todetermine when to open and close the steam valve.
 4. The systemaccording to claim 2 wherein the tubes include an inlet end and anoutlet end and the system includes at least one temperature measuringdevice near the inlet end of the tubes and at least one temperaturemeasuring device near the outlet end and the control system includeslogic to respond to predetermined changes in measured temperaturedifferences to determine when to open and close the steam valve.
 5. Thesystem according to claim 1 wherein the source of the heating fluid andthe source of the steam are the same source and steam is used to heatthe fluid by conduction and also remove foulant by periodic burstsinside the tube-side chamber.